Landing v2: icon switchers, ephemeral theme, channel link, drop browser CTA

Owner review-pass rework of the landing page:
- Rename the per-language Telegram link build var
  VITE_TELEGRAM_LINK_EN/_RU -> VITE_TELEGRAM_GAME_CHANNEL_NAME_EN/_RU
  (it carries a channel username; the landing builds https://t.me/<name> --
  the same channels the connector posts to via TELEGRAM_GAME_CHANNEL_ID_*).
- Language switcher -> a globe icon dropdown (flags + names), saved + synced
  to the app prefs.
- Theme switcher -> a sun/moon icon toggle, ephemeral (follows the system
  scheme, no auto, never persisted) -- galaxy-game style.
- Drop the "Play in browser" CTA (no standalone-web onboarding yet).

Docs: FUNCTIONAL(+ru), PLAN, deploy + ui READMEs.

gateway/Dockerfile

@@ -20,14 +20,14 @@ RUN corepack enable && corepack prepare pnpm@11.0.9 --activate
 # VITE_APP_VERSION carries `git describe` for the About screen (defaults to "dev").
 ARG VITE_TELEGRAM_BOT_ID=
 ARG VITE_TELEGRAM_LINK=
-ARG VITE_TELEGRAM_LINK_EN=
-ARG VITE_TELEGRAM_LINK_RU=
+ARG VITE_TELEGRAM_GAME_CHANNEL_NAME_EN=
+ARG VITE_TELEGRAM_GAME_CHANNEL_NAME_RU=
 ARG VITE_GATEWAY_URL=
 ARG VITE_APP_VERSION=
 ENV VITE_TELEGRAM_BOT_ID=$VITE_TELEGRAM_BOT_ID \
     VITE_TELEGRAM_LINK=$VITE_TELEGRAM_LINK \
-    VITE_TELEGRAM_LINK_EN=$VITE_TELEGRAM_LINK_EN \
-    VITE_TELEGRAM_LINK_RU=$VITE_TELEGRAM_LINK_RU \
+    VITE_TELEGRAM_GAME_CHANNEL_NAME_EN=$VITE_TELEGRAM_GAME_CHANNEL_NAME_EN \
+    VITE_TELEGRAM_GAME_CHANNEL_NAME_RU=$VITE_TELEGRAM_GAME_CHANNEL_NAME_RU \
     VITE_GATEWAY_URL=$VITE_GATEWAY_URL \
     VITE_APP_VERSION=$VITE_APP_VERSION